This invention relates in general to the detection of liquid leakage and deals more particularly with the detection of leakage from underground fuel storage tanks and the delivery lines through which the fuel is pumped for dispensing.
In gasoline service stations, it is common practice for automotive fuel to be stored in large underground tanks from which it is pumped through product lines to various types of dispensers. Typically, the dispenser is a pedestal equipped with a dispensing nozzle and a switch which is turned on to open a solenoid valve that permits the fuel to be dispensed. The nozzle is provided with a hand operated valve which opens and closes the nozzle to control the dispensing of fuel. Each pedestal may have a number of different nozzles and switches for dispensing different types of fuels such as regular gasoline, unleaded gasoline, diesel fuel, and one or more premium fuel grades.
The fuel is pumped from the storage tank to the dispenser by a submersible pump which is activated when the dispenser switch is turned on and deactivated at the end of the dispensing operation. Normally, a check valve is incorporated in the pump assembly so that at the end of a dispensing cycle, the product line retains the fuel that has been pumped into it, thus permitting the gasoline to be delivered immediately at the start of the next dispensing cycle. A pressure relief valve built into the pump assembly is used to relieve the product line pressure to a level of 11-15 psi following the end of each dispensing cycle.
Because of the positive pressure that is maintained in the product line, any leaks in the piping between the check valve and the dispenser can result in significant amounts of fuel leaking from the product line and contaminating the subsoil and groundwater. In addition, leaks in the underground tank can develop and result in the loss of considerable amounts of fuel into the ground. Such leaks have been of increasing concern in recent years due in part to the significant monetary losses that can be incurred due to the leakage of increasingly higher priced fuels. Perhaps even more importantly, the increasing emphasis that has been placed on environmental concerns in recent years has prompted industry standards and proposed laws and regulations that limit the amount of leakage that is considered acceptable.
The problem of tank leakage can be dealt with by using a double wall tank in a so called secondary containment system. The second or outer container serves the purpose of confining any fuel that may leak from the primary tank. However, the expense of a double wall tank is considerably greater than that of a single tank, and it is economically impractical in most situations to replace an existing single tank installation with a double wall tank. Dealing with leakage from the product lines presents in many ways an even more difficult problem, due largely to the difficulty that is involved in detecting leakage from the product line. The approaches that have been taken in the past to the detection of leaks in pressurized piping have generally relied on monitoring of the pressure in the system and interpreting large pressure drops as being caused by a leak in the piping. However, thermal effects can be and often are influential enough to create pressure drops that give false leak indications. For example, if relatively warm gasoline is pumped into a cold discharge pipe, the liquid undergoes a thermally induced volume contraction which can reduce the pressure in the pipe significantly even though there is no leakage of fluid. Consequently, unless a detector is able to distinguish pressure losses caused by a leak from those caused by thermal contraction, it will not be able to reliably detect a true leak in the piping system.
The problem of distinguishing between thermal contraction and actual leakage is compounded by the many variables that influence thermal decay. Among these variables are the gasoline blends and seasonal blend changes, the gasoline temperature when delivered to the underground tank, the speed with which the tank is depleted, the characteristics of the discharge piping such as its material, size, length and number of parallel lines, the type of back fill materials around the pipes, the groundwater level, the presence or absence of air in the product lines, and the ground temperature as effected by the climate, season, weather patterns, station pad material and the presence or absence of shading. Because of the uncertainties caused by the possible variations in these and other factors that can effect the thermal response of the liquid in the system, there have been no leak detectors available in the past that have been able to successfully distinguish between thermal contraction and leakage in order to provide reliable detection of true leaks.